1. Field of the Invention
The present invention describes displacement measurement methods and displacement measurement equipments that allow non-destructive and quantitative measurements of internal mechanical properties or internal physical quantities such as a displacement, a strain, a velocity, an acceleration etc in various objects, structures, substances, materials, living tissues etc. For instance, the present invention includes methods and equipments that generate an ultrasound echo by performing a proper beamforming such that at least one internal distribution of a displacement vector, a strain tensor, a strain rate tensor, a velocity vector and an acceleration vector can be measured. Such physical quantities are generated by a mechanical source such as an external pressure, an external vibration, a radiation force etc. The methods and equipments can also deal with tissues that move spontaneously such as a heart and a lung etc or moved by a body motion, a respiratory, a heart motion, a pulsation etc. Also those can also deal with a blood flow in a heart or a blood vessel. A contrast medium can also be used. Simultaneous measurements of such a tissue motion and a blood flow can also be performed. The results measured can be applied widely, for instance, to measurements of mechanical properties and thermal properties.
A medical field is a typical field to which the present invention is applied, such as ultrasonic diagnosis equipments, magnetic resonance imaging equipments, optical diagnosis equipments etc and radiotherapies etc. That is, for instance, deformability and motion-ability of tissues and blood can be examined for diagnoses. Otherwise, by performing a tracking of a target tissue motion, the safety, reliability and efficiency of various treatments can be increased. Degeneration can also be monitored for confirming a treatment effectiveness of a region of interest (ROI). Application of the present invention is not limited to these. For instance, as non-destructive measurement methods, the evaluations, examinations and diagnoses can be performed for various objects.
2. Description of a Related Art
For instance, in a medical field, treatments of diseases are performed by a radiotherapy, applications of a high intensity focus ultrasound, a leaser, an electromagnetic radio frequency wave or a microwave, a cryotherapy, or a cooling therapy etc. In these cases, an above-mentioned tracking and a non-invasive monitoring of the treatment effectiveness can be performed. Also an effectiveness of a medicine such as an anti-cancer drug can be performed non-invasively. For instance, the treatment effectiveness can be monitored by non-invasively measuring the degeneration and temperature change generated by radiotherapies. Otherwise, the observation of blood flow in a lesion allows the differentiation of the progress of disease. In order to perform a diagnosis and observe a treatment effectiveness, a tissue characterization can also be performed by evaluating an elastic constant etc after the measurements of displacement, strain and their spatial and temporal changes etc generated by forces applied in a lesion and a treated part.
It is well known that the temperature of a tissue has correlations with the elastic constants, visco-elastic constants, a delay time and a relaxation time with respect the constants, a density etc. Then, by performing non-destructive measurements of elastic constants such as a shear modulus and a bulk-modulus etc, visco-elastic constants such as a visco-shear modulus and a visco-bulk modulus etc, a time delay and a relaxation time with respect the moduli, a density etc, the temperature of a point of interest (POI) and the temperature distribution in an ROI can be measured. The temperature can also be estimated from the strains measured generated by a temperature change. Such temperature measurements allow the monitoring of thermal treatments and the prediction of a temperature distribution to be generated for a planning of the thermal treatments.
An ultrasound diagnosis equipment used in a medical field uses an ultrasound transducer for transmitting an ultrasound into a tissue and receiving ultrasound echo signals generated in the tissue by a reflection or scattering. The received echo signals are converted into an ultrasound image observable, which exhibits a distribution of tissues. Then, the measurements of tissue displacement (vector) generated by an arbitrary mechanical source, tissue strain (tensor) generated, elastic constants etc using such an equipment allows the non-invasive observation of the differences between a lesion and a normal tissue.
At past, for the displacement distribution measurement, the change of echo signals obtained by transmitting ultrasound at different phases (plural phases or times) is observed. From the measured displacement, the strains etc is estimated. Concretely, it is proposed that three, two or one-dimensional ROI is set in a target tissue, and a distribution of three, two or one component of a three dimensional displacement vector is measured. Then, elastic constants etc in ROIs are evaluated from the measured displacements, strains etc.
The transducer is a sensor of a displacement or strain measurement. Instead of the ultrasound transducer, other known transducers such as electromagnetic sensors, light sensors, laser sensors can also be used. The sensor is a contact type or non-contact type. For a mechanical source that yields a displacement or a deformation, the ultrasound transducer itself can also be used as a static compressor. The transducer can also be used as a vibrator by assembling a mechanical vibration function into the transducer contact surface. Also others from the transducer, a compressor or a vibrator can be used. A heart motion or a pulsation can also be used (i.e., internal mechanical sources). Also a radiated ultrasound or vibration from a transducer can be used as a mechanical source to yield a deformation in an ROI. The transducer can work as a sensor as well. For the tissue characterization, change in elastic constants, temperatures, thermal properties etc generated by a treatment can also be used.
However, in the most classical fashion, a tissue displacement is measured by applying a one-dimensional signal processing to the ultrasound echo signals under the assumption that the target tissue moves in the same direction as that of the ultrasound beam. Therefore, when the target tissue displacement (motion) has a lateral component (component of the orthogonal direction to the beam direction), the measurement accuracy of the beam direction (axial direction) becomes low (ref. 1: C. Sumi et al, “Phantom experiment on estimation of shear modulus distribution in soft tissue from ultrasonic measurement of displacement vector field”, IEICE Trans. on Fundamental, vol. E78-A, no. 12, pp. 1655-1664, December 1995). Here, the ultrasound echo signals involve a raw echo signal, an analytic signal, a quadrate detection, and an envelope detection etc. It is also impossible to measure the lateral displacement component. Therefore, there exists a limitation for the measurement accuracy of the blood flow in a heart and a blood vessel running parallel to the body surface. In addition, it is also difficult to deal with a part of which deformation cannot be controlled externally and a tissue deforms spontaneously by the internal mechanical sources such as a heart etc (for instance, a liver).
Alternatively, the present inventor proposes a displacement vector measurement method that yields the vector measurement from the gradient of the phase of local multidimensional (three- or two-dimensional) cross-spectrum of echo signals (multidimensional cross-spectrum phase gradient method). The cross-correlation method can also be used (refs. 1 and 2: C. Sumi, “Fine elasticity imaging on utilizing the iterative rf-echo phase matching method,” IEEE Trans. on Ultrasonics, Ferroelectrics and Frequency Control, vol. 46, no. 1, pp. 158-166, January 1999, etc). The present inventor also propose a multidimensional autocorrelation method and multidimensional Doppler method that deal with multidimensional analytic signals (ref. 3: C. Sumi, “Digital measurement method of tissue displacement vector from instantaneous phase of ultrasonic echo signal,” Technical report of Japan Society of Ultrasound Medicine, pp. 37-40, December 2002, Tokyo, Japan (in Japanese) or C. Sumi, “Displacement vector measurement using instantaneous ultrasound signal phase—Multidimensional autocorrelation and Doppler methods,” IEEE Trans. on Ultrasonics, Ferroelectrics and Frequency Control, vol. 55, pp. 24-43, January 2008, etc). These measurement methods are properly used or properly combined according to the application of the measurement, specifically, according to the measurement accuracy and calculation time required.
For the displacement vector measurement, the multidimensional phase matching method which the present inventor previously invented is effective (refs. 1 to 3). The phase matching is performed in a multidimensional space (a three-dimensional space is expressed using axial, lateral and elevation directions; a two-dimensional region is expressed using axial and lateral directions) for the same paired echo signals by rotating a phase of the 1st or 2nd echo signals in an analogue manner using the measured displacements, or by spatially shifting the local echo signals using the approximated displacement components as a multiplication of the corresponding sampling intervals by truncating or round off. The phase matching prevents the measurement from suffering an aliasing due to a large displacement in a beam direction. Also the phase matching increases a measurement accuracy of all the displacement components (i.e. all directions) by increasing a correlation and/or coherence between the echo signals. For instance, coarse measurements obtained by the multidimensional cross-correlation method or the multidimensional cross-spectrum phase gradient method are used to perform a coarse phase matching. After the coarse phase matching, the multidimensional cross-spectrum phase gradient method, the multidimensional autocorrelation method or the multidimensional Doppler method is used to yield a fine measurement. Otherwise, after the coarse phase matching, the corresponding one-dimensional methods can also be applied to yield only an axial displacement measurement or a displacement vector components, although the measurement accuracy is lower than that obtained by the multidimensional methods (ref. 3).
By performing the phase matching, even if an uncontrollable mechanical source exists in an ROI (heart motion, respiratory, blood vessel pulsation, body motion etc), it is also possible to measure a displacement vector or an axial displacement. Thus, the phase matching allows yielding the useful measurement absolutely. The multidimensional vector measurement methods also yield results in a real-time similarly to the corresponding one-dimensional methods.
However, even if the phase matching is performed, the measurement accuracy of the axial displacement becomes low when using the one-dimensional displacement measurement method. This is because the residual lateral displacement exists. Thus, the measurement accuracy depends on the accuracy of the coarse phase matching. When using a conventional ultrasound equipment for the displacement vector measurement, the accuracy and spatial resolution of the measured lateral displacement are low, because the lateral carrier frequency does not exist and the lateral bandwidth is smaller than the axial one. Thus, the measurement accuracy of a displacement vector and a strain tensor becomes low, being dependent of the measurement accuracy of the lateral displacement.
Then, the present inventor realized a remarkably accurate displacement vector measurement on the basis of the above-mentioned displacement vector measurement methods, however, with a use of echo signals having lateral and elevational carrier frequencies. Such a use of echo signals allows the increase in a measurement accuracy of an axial displacement. It is referred to as a lateral modulation we called (refs. 3 and 4: C. Sumi et al, “Effective lateral modulations with applications to shear modulus reconstruction using displacement vector measurement,” IEEE Trans. on Ultrasonics, Ferroelectrics and Frequency Control, vol. 55, pp. 2607-2625, Dec. 2008; ref. 5: C. Sumi et al, “Comparison of parabolic and Gaussian lateral cosine modulations in ultrasound imaging, displacement vector measurement, and elasticity measurement”, Jpn, J. Appl. Phys., vol. 47(5B), pp. 4137-4144, May 2008 etc). For the lateral modulation, J. A. Jensen et al and M. E. Anderson determined an apodization function to be used using the Fraunhofer approximation (ref. 6: J. A. Jensen, “A new method for estimation of velocity vectors”, IEEE Trans. Ultrason., Ferroelect., Freq. Contr., vol. 45, pp. 837-851, 1998; ref. 7: M. E. Anderson, “multi-dimensional velocity estimation with ultrasound using spatial quadrature”, IEEE Trans. Ultrason., Ferroelect., Freq. Contr., vol. 45, pp. 852-861, 1998), whereas the present inventor determines the apodization function using the optimization method developed (ref. 8: C. Sumi et al, “A demonstration of optimal apodization determination for proper lateral modulation”, Jpn, J. Appl. Phys., vol. 48(7B), July 2009, etc).
For the beamforming of a lateral modulation, the present inventor realized various useful methods on the basis the use of crossed, steered beams (i.e., not the Fraunhofer approximation, ref. 4): using a classical monostatic or multistatic synthetic aperture (SA) method; transmitting crossed beams simultaneously and receiving crossed beams simultaneously; transmitting crossed plane beams for a wide region and superposing generated echo beams; using such crossed beams, however, transmitted or received using different or plural physical apertures; using such crossed beams, however, generated at different phases.
The lateral modulation developed by the present inventor can be realized by achieving crossed beams using steered beams in arbitrary directions. Then, arbitrary type transducers such as a linear array type transducer can be used, i.e., arbitrary coordinate systems can be used. In such coordinate systems, crossed beams can be generated using steered beams having arbitrary directions. Thus, the crossed beams are not always symmetric in a lateral direction. Such a non-symmetric beamforming is effective when the obstacles such as a bone exist, for instance. Non-steered beam can also be used as one beam of crossed beams. Mechanical steering can also be used together. At a point of interest (POI), a steering angle or crossed angle of steered beams should be large. Although the lateral modulation can be performed with a consideration about the apodization (beam shape), the apodization optimized by the present inventor allows yielding a lateral modulation with a wide lateral bandwidth. This increases the spatial resolution of ultrasound image obtained and the measurement accuracy of a displacement vector measurement achieved.
For the lateral modulation (ref. 3), to obtain a three-dimensional echo data frame (i.e., three-dimensional displacement vector measurement), crossed beams must be generated using four or three steered beams, whereas to obtain two-dimensional echo data frame (i.e., two-dimensional displacement vector measurement), crossed beams must be generated using two steered beams. In the present invention, similarly to a conventional case, the respective three- or two-dimensional frames generated are approximated as echo data frames that exhibit a tissue distribution at respective time phases, although it takes a finite time to receive all echo signals and generate the respective frames. The inversion of the time difference between the frames obtained is called as a frame rate. Due to the existence of a tissue motion, the time required to generate one frame should be short. Hereafter, the time phase is referred to as defined here.
For the lateral modulation, however, larger number of beams must be generated than the conventional beamforming. Then, it may take a longer time to obtain echo signals and achieve the signal processing such as the apodization, switching, implementation of delay on echo signals, phase matching on echo signals, summation of echo signals. Then, the frame late may become lower than the conventional imaging. When using a classical synthetic aperture (SA), the echo signal-to-noise ratio (SNR) obtained will be low, because the transmitted ultrasound powers from the respective elements are small. In addition, particularly when dealing with deeply situated tissues, a larger physical aperture is required than that for a conventional beamforming. The field of vision (FOV) obtained may become smaller in lateral and elevational directions than that for a conventional beamforming.
Alternatively, an accurate displacement vector measurement can be achieved by synthesizing the accurately measured axial displacements obtained from respective steered beams with the same steering angle (ref. 3), i.e., the superposition is not performed. However, the frame rate may become low similarly to the lateral modulation. When using steered beams obtained at different phases, the tissue displacement between the two frames obtained leads to a measurement error.